1. Field of the Invention
The present application generally relates to systems and methods for creating ablation zones in human tissue. More specifically, the present application relates to transducer configurations used to create tissue lesions, and even more specifically to ultrasound transducers used to treat fibrillation of the heart. While the present application emphasizes treatment of atrial fibrillation, one of skill in the art will appreciate that this is not intended to be limiting, and that the systems and methods disclosed herein may also be used to treat other arrhythmias as well as to treating other conditions by creating lesions in tissue.
The condition of atrial fibrillation (AF) is characterized by the abnormal (usually very rapid) beating of the left atrium of the heart which is out of synch with the normal synchronous movement (‘normal sinus rhythm’) of the heart muscle. In normal sinus rhythm, the electrical impulses originate in the sino-atrial node (‘SA node’) which resides in the right atrium. The abnormal beating of the atrial heart muscle is known as ‘fibrillation’ and is caused by electrical impulses originating instead at points other than the SA node, for example, in the pulmonary veins (PV).
There are pharmacological treatments for this condition with varying degree of success. In addition, there are surgical interventions aimed at removing the aberrant electrical pathways from PV to the left atrium (‘LA’) such as the ‘Cox-Maze III Procedure’. This procedure has been shown to be 99% effective but requires special surgical skills and is time consuming. Thus, there has been considerable effort to copy the Cox-Maze procedure using a less invasive percutaneous catheter-based approach. Less invasive treatments have been developed which involve use of some form of energy to ablate (or kill) the tissue surrounding the aberrant focal point where the abnormal signals originate in PV. The most common methodology is the use of radio-frequency (‘RF’) electrical energy to heat the muscle tissue and thereby ablate it. The aberrant electrical impulses are then prevented from traveling from PV to the atrium (achieving the ‘conduction block’) and thus avoiding the fibrillation of the atrial muscle. Other energy sources, such as microwave, laser, and ultrasound have been utilized to achieve the conduction block. In addition, techniques such as cryoablation, administration of ethanol, and the like have also been used. Some of these methods and devices are described below.
There has been considerable effort in developing catheter based systems for the treatment of AF using radiofrequency (RF) energy. One such method includes a catheter having distal and proximal electrodes at the catheter tip. The catheter can be bent in a coil shape, and positioned inside a pulmonary vein. The tissue of the inner wall of the PV is ablated in an attempt to kill the source of the aberrant heart activity.
Another source used in ablation is microwave energy. One such intraoperative device consists of a probe with a malleable antenna which has the ability to ablate the atrial tissue.
Still another catheter based method utilizes the cryogenic technique where the tissue of the atrium is frozen below a temperature of −60 degrees C. This results in killing of the tissue in the vicinity of the PV thereby eliminating the pathway for the aberrant signals causing the AF. Cryo-based techniques have also been a part of the partial Maze procedures described above. More recently, Dr. Cox and his group have used cryoprobes (cryo-Maze) to duplicate the essentials of the Cox-Maze III procedure.
More recent approaches for the treatment of AF involve the use of ultrasound energy. The target tissue of the region surrounding the pulmonary vein is heated with ultrasound energy emitted by one or more ultrasound transducers. One such approach includes a catheter distal tip portion equipped with a balloon and containing an ultrasound element. The balloon serves as an anchoring means to secure the tip of the catheter in the pulmonary vein. The balloon portion of the catheter is positioned in the selected pulmonary vein and the balloon is inflated with a fluid which is transparent to ultrasound energy. The transducer emits the ultrasound energy which travels to the target tissue in or near the pulmonary vein and ablates it. The intended therapy is to destroy the electrical conduction path around a pulmonary vein and thereby restore the normal sinus rhythm. The therapy involves the creation of a multiplicity of lesions around individual pulmonary veins as required.
Yet another catheter device using ultrasound energy includes a catheter having a tip with an array of ultrasound elements in a grid pattern for the purpose of creating a three dimensional image of the target tissue. An ablating ultrasound transducer is provided which is in the shape of a ring which encircles the imaging grid. The ablating transducer emits a ring of ultrasound energy at 10 MHz frequency.
While such ablation therapies alone are promising, it is preferred that devices and systems combine these ablation therapies with imaging capabilities in a single unit. It would be particularly useful to provide sensing or imaging (often used interchangeably) of the treatment region to properly position the ablation device relative to the treatment region, as well as to evaluate progression of the treatment. Such imaging assists the system or the operator to ensure that only the targeted tissue region is ablated. Furthermore, in a moving target such as heart tissue, the original target identified by imaging, can move and thus non-target tissue may be inadvertently ablated. Hence, contemporaneous (or almost contemporaneous) imaging and ablation minimizes the risk of ablating non-target tissue. Thus, one unmet need using ultrasound techniques for tissue ablation is to provide a device capable of both imaging as well as ablation.
Attaining this goal involves redesigning the key components of a conventional ultrasound ablation system to also provide an imaging function. Typically, ultrasound ablation is accomplished using a transducer assembly. The transducer assembly comprises a transducer element, commonly one or more piezoelectrically active elements such as lead zirconate titanate (PZT) crystals. The PZT crystals often include an acoustical (impedance) matching layer on the ablating face to facilitate efficient power transmission and to improve the imaging performance. Further, the crystals may be bonded to a backing on the non-ablative face to reflect or absorb any ultrasound beams in the appropriate direction. The conventional acoustic transducers which are typically employed for the therapeutic purposes are acoustically large, often single-crystal devices having a narrower bandwidth in the frequency domain than is required for good imaging performance. Although they are designed to efficiently transmit acoustic energy to the target tissue, crystal devices with narrow bandwidth have previously been viewed as unsuited for imaging. This has been due to the perceived inability of conventional ablation transducers to handle the bandwidth of the ultrasound frequencies that would be optimized for both imaging and ablation. While ablation can be achieved using a narrower range of frequencies, imaging is usually performed using a wide range of frequencies. Thus, it is desirable that the PZT be able to accommodate a wider bandwidth than used for ablation in order to accommodate the imaging bandwidth.
Wider transducer bandwidths are often achieved through the use of matching layers. Matching layers typically use materials with acoustic impedance between the acoustic impedances of the PZT and the tissue, and with a thickness approaching ¼ wavelength of the ultrasound frequency utilized. While matching layers are often used to improve the transmission of ultrasound from the PZT into the tissue, they also can be used to dampen the mechanical response of the PZT and broaden its bandwidth. This dampening can result in some reduction of transducer efficiency. Furthermore wide bandwidth transducers may be unable operate at high power levels because they cannot be cooled effectively, partly due to the thermally insulating properties of the matching layer. A conventional PZT transducer with a higher bandwidth may often be only 30%-50% efficient in converting the electrical energy to acoustic energy, and much of the energy is converted to heat and lost in the transducer assembly. In addition to the lack of efficiency in converting to ultrasound energy, the heat further reduces the PZT efficiency and may cause the PZT crystal to depole and stop functioning as a transducer.
Thus, an additional challenge is to cool the transducer to maintain a lower operating temperature than is presently provided for in commercially available systems. A cooled transducer can be driven harder, i.e., it can tolerate higher electric powers and produce higher acoustic powers. This higher acoustic output is useful in increasing the lesion size and/or reducing the amount of time required to create a lesion. Both of these attributes are important in the clinical application of treating AF.
One method of cooling the transducer is to take advantage of the power density and heat dissipation that are dependent on the size of the transducer. As the diameter (and corresponding surface area) of the transducer increases, the power density drops, and the heat dissipation per unit surface area also drops. If large enough, conventional cooling methods may suffice to keep the transducer cool. However, in a catheter suitable for ablation using an interventional approach, the transducer must necessarily be small and yet also be able to generate the power density levels required to ablate tissue. In such a transducer, size is not a suitable method of regulating the transducer's temperature. Thus, due to the small transducer size and consequent high power densities and low heat dissipation, alternative approaches are warranted for cooling the transducer.
One potential solution is the use of fluids to cool the transducer. Commonly, bodily fluids, such as blood flowing around the transducer, are used as a cooling fluid. However, blood tends to denature and collect around the transducer when heated. In addition to the attendant problems of possibly creating a clot in the atrium, the denatured blood may also adhere to the face of the transducer and create a layer of insulation, thereby further decreasing the performance of the transducer. In contrast, introduced (non-bodily) fluids such as saline or water do not have the same attendant problems as blood and are useful in maintaining lower transducer operating temperatures. However, in order to be effective, these introduced fluids have to be effectively transported to the entire transducer to cool all the faces of the transducer. If fluid transport is inadequate, the uncooled regions may develop “hot spots” that can impede the efficiency of the transducer.
While some devices, such as single crystal ultrasound therapy systems have been reported for both imaging and therapeutic purposes, none disclose a method for cooling the entire transducer. Other multi-crystal transducer assemblies are also available that circumvent the concerns of the single-crystal model. Some of these systems provide a method for cooling the back of the transducer crystals. However, none of these systems or methods include cooling of the entire transducer crystal. As mentioned above, it is important to cool all the faces of the transducer (front and the back). Cooling only part of the transducer may lead to “hot spots” on some areas of the transducer, thereby decreasing the efficiency of the transducer in a situation where both ablation and imaging are necessary.
To realize combined imaging and ablation capabilities, some systems have separate imaging and ablation units. For example one commercially available system includes a treatment and imaging system. This system comprises a probe with an ultrasound transducer adapted to obtain imaging information from a patient treatment region, and also a separate arm member to deliver ultrasonic energy to the treatment region. Naturally, these are bulky and not well suited for use in catheter based systems. A variant of the combined imaging and ablation units is using separate transducer elements for imaging and ablation. This approach suffers from many shortcomings including functionally, the ablated tissue is not identical to the imaged tissue, and structurally this configuration of discrete imaging and ablating elements occupies more space in a housing, where space is limited in a transducer assembly, especially when the transducer is at the tip of the catheter as used in an interventional approach. Additionally, a multi-element device is more expensive and inconvenient to manufacture, along with the complicated arrangements necessary for cooling the transducer elements. Further, multi-element devices are prone to misalignment, which may make them more difficult to use. Also, multi-element devices typically require more complex and expensive systems for their control and use.
Thus, additional improvements are still desired in the field of ultrasound devices with combined imaging and ablating capabilities. In particular, it would be desirable to provide a device with a single-crystal transducer assembly where all faces of the transducer crystal are cooled to protect and preserve the operating efficiency. It would also be desirable to provide such a system that is easy to use, easy to manufacture and that is lower in cost than current commercial systems.
2. Description of Background Art
Patents related to the treatment of atrial fibrillation include, but are not limited to the following: U.S. Pat. Nos. 7,393,325; 7,142,905; 6,997,925; 6,996,908; 6,966,908; 6,964,660; 6,955,173; 6,954,977; 6,953,460; 6,949,097; 6,929,639; 6,872,205; 6,814,733; 6,780,183; 6,666,858; 6,652,515; 6,635,054; 6,605,084; 6,547,788; 6,514,249; 6,502,576; 6,500,121; 6,416,511; 6,383,151; 6,305,378; 6,254,599; 6,245,064; 6,164,283; 6,161,543; 6,117,101; 6,064,902; 6,052,576; 6,024,740; 6,012,457; 5,629,906; 5,405,346; 5,314,466; 5,295,484; 5,246,438; 4,757,820 and 4,641,649.
Patent Publications related to the treatment of atrial fibrillation include, but are not limited to International PCT Publication Nos. WO 2005/117734; WO 1999/002096; and U.S. Patent Publication Nos. 2005/0267453; 2003/0050631; 2003/0050630; and 2002/0087151.
Scientific publications related to the treatment of atrial fibrillation include, but are not limited to: Haissaguerre, M. et al., Spontaneous Initiation of Atrial Fibrillation by Ectopic Beats Originating in the Pulmonary Veins, New England J Med., Vol. 339:659-666; J. L. Cox et al., The Development of the Maze Procedure for the Treatment of Atrial Fibrillation, Seminars in Thoracic & Cardiovascular Surgery, 2000; 12: 2-14; J. L. Cox et al., Electrophysiologic Basis, Surgical Development, and Clinical Results of the Maze Procedure for Atrial Flutter and Atrial Fibrillation, Advances in Cardiac Surgery, 1995; 6: 1-67; J. L. Cox et al., Modification of the Maze Procedure for Atrial Flutter and Atrial Fibrillation. II, Surgical Technique of the Maze III Procedure, Journal of Thoracic & Cardiovascular Surgery, 1995; 110:485-95; J. L. Cox, N. Ad, T. Palazzo, et al. Current Status of the Maze Procedure for the Treatment of Atrial Fibrillation, Seminars in Thoracic & Cardiovascular Surgery, 2000; 12: 15-19; M. Levinson, Endocardial Microwave Ablation: A New Surgical Approach for Atrial Fibrillation; The Heart Surgery Forum, 2006; Maessen et al., Beating Heart Surgical Treatment of Atrial Fibrillation with Microwave Ablation, Ann Thorac Surg 74: 1160-8, 2002; A. M. Gillinov, E. H. Blackstone and P. M. McCarthy, Atrial Fibrillation: Current Surgical Options and their Assessment, Annals of Thoracic Surgery 2002; 74:2210-7; Sueda T., Nagata H., Orihashi K., et al., Efficacy of a Simple Left Atrial Procedure for Chronic Atrial Fibrillation in Mitral Valve Operations, Ann Thorac Surg 1997; 63:1070-1075; Sueda T., Nagata H., Shikata H., et al.; Simple Left Atrial Procedure for Chronic Atrial Fibrillation Associated with Mitral Valve Disease, Ann Thorac Surg 1996; 62:1796-1800; Nathan H., Eliakim M., The Junction Between the Left Atrium and the Pulmonary Veins, An Anatomic Study of Human Hearts, Circulation 1966; 34:412-422; Cox J. L., Schuessler R. B., Boineau J. P., The Development of the Maze Procedure for the Treatment of Atrial Fibrillation, Semin Thorac Cardiovasc Surg 2000; 12:2-14; and Gentry et al., Integrated Catheter for 3-D Intracardiac Echocardiography and Ultrasound Ablation, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 51, No. 7, pp 799-807.